Apolipoprotein E (apoE) is a polymorphic, multifunctional protein synthesized by several cell types and tissues, including liver, kidney, skin, adipose tissue, macrophages and brain. The wide distribution of apoE is associated with the maintenance of key cellular functions such as intracellular cholesterol trafficking, cholesterol distribution between cells, and tissue reparation.
The amino acid sequence of the apoE protein is well conserved throughout species. ApoE can be viewed as a regulator of cholesterol homeostasis in tissues such as the central nervous system (CNS) and peripheral nervous system (PNS) and the arterial wall (cell-cell) or between tissues via the circulating plasma lipoproteins (tissue-tissue).
The major role of plasma apoE containing lipoproteins is to transfer lipids (cholesterol) from peripheral tissues to the liver and to remove excess cholesterol from peripheral tissues via the reverse cholesterol transport system. Dysregulation of this mechanism leads to excess cholesterol deposition in peripheral tissues such as arteries (arterosclerosis) and skin (xanthomas and xanthelasmas). ApoE has also been shown to have a direct effect on lymphocyte proliferation and thus has an immunomodulatory role.
ApoE is the only lipoprotein synthesized in the brain and has a key role in cholesterol transport between cells of the CNS. Local secretion of apoE by cells such as macrophages or macrophage-derived cells is essential for the uptake of excess tissue cholesterol and the provision of cholesterol for specific needs such as nerve repair and remyelinisation.
Up to the present time, compounds affecting apoE production in vitro and in vivo have not been extensively investigated. Only hormone-like estrogens and corticoids have been shown to change apoE levels under various experimental conditions (Srivastava et al., 1997; Stone et al., 1997).
There is currently a need for compounds that modulate apoE synthesis and secretion, such compounds having application in the treatment of diseases such as atherosclerosis, excess lipid deposition in peripheral tissues such as skin (xanthomas), stroke, memory loss, optic nerve and retinal pathologies (i.e., macular degeneration, retinitis pigmentosa), repair of traumatic damage of the central nervous system (brain tissue), repair of traumatic damage of the peripheral nervous system (i.e., nerve section compression or crush), prevention of the degenerative process due to aging (i.e., Alzheimer's disease), prevention of degenerative neuropathies occurring in diseases such as diabetic neuropathies and multiple sclerosis, autoimmune diseases and activation of the innate immune system.